In the synthesis of polyalkenes such as polyethylene and polypropylene, solid particles of polymer product are withdrawn from the reactor with interparticle gaseous components present in the reactor. The solid polymer product is purged with an inert gas such as nitrogen before the product is pelletized, yielding a nitrogen-rich purge gas containing a number of hydrocarbon compounds including unreacted ethylene or propylene, some alkanes such as ethane or propane, comonomers such as butene-1 and hexene-1, additives such as isobutane or isopentane which are added to the reactor feed as diluents or moderators, and small amounts of oligomers such as octene, decene, or heavier olefins. The hydrocarbon content of this product purge gas can range up to 50 mol% or higher, most of which consists of the valuable feed components ethylene or propylene. Recovery of these feed components is economically desirable, as is recovery of the comonomers and additives if present. Significant amounts of heavier components such as oligomers, if present in the purge gas, will be detrimental to adsorption or low temperature condensation methods for recovering the valuable lighter components.
Adsorption processes have been proposed to separate hydrocarbons from purge or vent gas streams produced by processes similar to polyethylene and polypropylene reactor systems. U.S. Pat. No. 3,266,221 discloses a pressure swing adsorption unit which uses molecular sieves to separate hydrocarbons, primarily ethylene and propylene, from byproduct carbon dioxide and diluent nitrogen in a direct oxidation process for the production of ethylene oxide. The recovered hydrocarbons are then compressed, purified and recycled to the reaction zone.
U.S. Pat. No. 4,769,047 discloses an improvement to that process whereby the ethylene oxide vent gas stream is first contacted with activated carbon to remove the propylene and higher hydrocarbons which have a deleterious effect when recycled to the ethylene oxide reactor with the ethylene. The ethylene is then recovered in a downstream pressure swing adsorption unit, purified by conventional liquid absorption methods to remove residual carbon dioxide, compressed and recycled to the ethylene oxide reactor system. Since heavy oligomers such as octene and decene are not formed in the ethylene oxide reactor system, there is no problem of permanent adsorption of these materials in the pressure swing adsorption units proposed in these patents. Potential problems with other heavy materials which might be present in the vent gases are not addressed in either of these patents.
Pressure swing adsorption can be utilized to recover ethylene and propylene from the nitrogen purge gas used in polyethylene and polypropylene production processes. Most of the ethylene and propylene can be removed from the nitrogen so that a lower quantity of hydrocarbons is discharged to the atmosphere with the nitrogen. However, the purity of the recovered ethylene and propylene is too low, that is, it contains too much nitrogen, to be recycled to the reactor system and instead must be burned as fuel or otherwise disposed of.
U.S. Pat. No. 3,336,281 discloses an improvement to a high pressure process for the manufacture of ethylene copolymers operating at 7350 psia or above in which ethylene off-gas from a high pressure copolymer product separator at 2940 psia or above is cooled to a temperature of 0.degree. C. to -65.degree. C. (+32.degree. F. to -85.degree. F.) by expansion to atmospheric pressure. As a result of this expansion cooling step, a large fraction of the undesirable low molecular weight by-products in the off-gas are condensed and removed and the uncondensed ethylene can be recycled to the reactor system. The improvement of this invention is directed primarily to the removal of the undesirable low molecular weight by-products from a recycle ethylene stream; the separation of ethylene from light gases such as nitrogen is not addressed. The potential problems of freeze-out of byproducts or comonomers in the cold equipment are not addressed in this patent.
U.S. Pat. No. 4,217,431 discloses an improvement to a high pressure process for the manufacture of ethylene copolymers operating at 20,000 to 75,000 psia in which off-gas from a low pressure copolymer product separator at 150 psia or less is compressed and cooled to -10.degree. C. to -30.degree. C. (+14.degree. F. to -22.degree. F.). As a result of this cooling step, a large fraction of the comonomer in the off-gas, such as vinyl acetate, is condensed and can be recycled to the reactor system. However, most of the ethylene in the off-gas, typically 60 to 70%, must be discharged from the system. The improvement of this invention is directed primarily to the recovery of the comonomer for reuse; the separation of ethylene from light gases is not addressed in the patent. The potential problems of freeze-out of byproducts or comonomers in the cold equipment are not addressed in this patent.
U.S. Pat. No. 5,233,060 discloses an absorption/stripping process to recover ethylene from argon purge gas in a direct oxidation process for the production of ethylene oxide. The ethylene is absorbed in a high molecular weight organic liquid, such as n-dodecane, and subsequently stripped from the liquid with nitrogen or methane. The stripped ethylene is then recycled to the reactor system, along with the nitrogen or methane stripping gas. Recycling of the nitrogen or methane stripping gas is acceptable in this ethylene oxide process since these gases are normally used as diluent or ballast gases in the reactor. Potential problems with accumulation of heavy materials in the scrub liquid (because these materials can only be partially stripped from the scrub liquid) are not addressed.
A paper by A. Fatemizadeh and E. Nolley presented to the American Institute of Chemical Engineers, Orlando, Fla., Mar. 19-23, 1990 entitled "Ethylene Recovery from Polyethylene Plant Vent Streams" describes in generic terms a cold box concept which uses expander refrigeration for ethylene recovery from purge gas streams containing less than 20 mole % nitrogen and other light gases. Specific details of the cold box process adequate removal of light gases. The process requires an expansion turbine and is limited to gas streams containing less than 20 mole % nitrogen. A conventional lean oil absorption process is described for recovering ethylene from such a stream in which ethylene-containing oil is regenerated and recirculated to the absorption tower.
Improved methods for the recovery of ethylene or propylene from nitrogen-rich purge gas in the production of polyethylene or polypropylene are economically desirable. The presence of heavier hydrocarbons in the purge gas can adversely affect adsorption or condensation processes used for light hydrocarbon recovery, and methods for removing such heavy hydrocarbons are needed to allow recovery of valuable ethylene or propylene. The invention disclosed below and defined in the claims which follow addresses these problems and offers improved purge gas treatment and light hydrocarbon recovery.